We plan to investigate functional relations between motor cortex cells and specific arm muscles by recording their activity in alert rhesus monkeys operantly conditioned to perform the following responses: (a) with the arm semiprone in a hinged cast the monkey will actively flex and extend the wrist and elbow against programmed loads; (b) with the arm held isometrically, the monkey will contract specific muscles in insolation or in combination, and (c) will generate bursts of cortical cell activity. Sensory responses to passive movements and load perturbations will also be tested. Response averages of unit and muscle activity, position and force will be compiled to determine the most consistent unit-muscle covariations. These will be directly tested by conditioning dissociation of correlated elements. To investigate the organization of corticomotoneuronal connections, cell and muscle activity will be cross-correlated with spike-triggered averages of rectified EMG activity. The extent of post-spike facilitation of motor units within a muscle and in different muscles will indicate the divergence of terminal connections of single CM cells. Interconnection of CM cells with each other and with motoneurons will be further elucidated by effects of microstimulation near CM cells - both on firing probability of forelimb motor units in behaving monkeys and on intracellularly recorded EPSP's evoked in motoneurons in acute studies. Histological confirmation of connections between cortical recording sites and cervical motoneurons will also be sought. We also plan to record activity of identified interneurons and motoneurons in cervical spinal cord of behaving monkeys. Cells will be characterized by responses to natural stimulation, and electrical stimulation of peripheral nerves and pyramidal tract, and responses during active and passive wrist movements.